1. Field of the Invention
This invention relates to dispersion compensation in optical networks. More specifically, the invention relates to the use of uniform fiber Bragg gratings to provide dispersion compensation. The wavelength of light for which the dispersion compensation is provided by a grating may be dynamically controlled.
2. Description of the Problem
Dispersion compensation is a well-known technique for increasing the usable bandwidth of long-haul fiber systems that suffer from limitations caused by pulse broadening due to chromatic dispersion. Without dispersion compensation, a pulse, upon propagation through a dispersive system, will accumulate a phase chirp, leading to a broadening of pulses in the time-domain. The broadening of pulses in a data string then produces overlap of adjacent pulses and resulting detection errors at the receiver. An example of such broadening is shown in FIGS. 1 and 2, where eye diagrams of a non-dispersed, and a dispersion-broadened data string are shown. An eye diagram is a synchronous measurement of the received signal, where signals from multiple bit intervals are plotted over one another. Undistorted transmission leads to "open eyes," where the binary zero and one signals are clearly separated during the measurement interval as in FIG. 1, while dispersion-induced broadening leads to "closed eyes" as in FIG. 2. U.S. Pat. No. 5,861,970 issued Jan. 19, 1999 provides a good discussion of dispersion in optical networks and is incorporated herein by reference.
Dispersion within an optical network can be measured in a number of ways. U.S. Pat. No. 5,717,510, issued Feb. 10, 1998, and U.S. Pat. No. 5,815,294, issued Sep. 29, 1998, address the monitoring of specific signal characteristics, and are incorporated herein by reference. The monitoring systems discussed in these patents, as well as some other known monitoring systems, require synchronous detection of data with subsequent processing. Such systems provide accurate results, but require large amounts of storage and processing hardware, making them complex and expensive.
Two methods for dispersion compensation are currently used in practical systems. In the first method, dispersion compensators, formed by long lengths of optical fiber (with dispersion properties that cancel the dispersion characteristics of the transmission channel), are placed at strategic points in the network to keep the dispersion within limits, and at the receiver to reduce accumulated dispersion. At the receiver, careful adjustment of fiber lengths is necessary to match these characteristics at a specific wavelength for a specific system. Adjustment is made by switching long lengths of fiber into or out of the compensator. The long lengths necessary for this component introduce signal attenuation, so an optical amplifier is typically needed to offset this loss.
The second currently employed method uses linearly chirped fiber Bragg gratings. Here, the period of a fiber grating is varied linearly with distance, with a mean period that approximately satisfies the Bragg condition for strong reflection at the optical wavelength of interest. The linearly-varying period imparts chromatic dispersion on the reflected pulse which can be used to compensate for accumulated dispersion in long-haul signal propagation. These components typically operate over a narrow bandwidth, and the amount of dispersion compensation, which depends on the grating length, chirp parameter, and index modulation depth, cannot be adjusted easily. Various aspects of the fabrication and use of chirped fiber Bragg gratings are described in U.S. Pat. No. 5,420,948, issued May 30, 1995; U.S. Pat. No. 5,602,949, issued Feb. 11, 1997; and U.S. Pat. No. 5,718,738, issued Feb. 17, 1998; all of which are incorporated herein by reference.
An alternative technique is the use of a uniform fiber Bragg grating in transmission. It is known that a pulse propagating at a wavelength just outside the reflection band of a grating structure undergoes significant chromatic dispersion. This effect was observed in the paper, F. Ouellette, Optics Letters, vol. 29, no. 32, pp. 4826-4829, 1990. Use of this technique for dispersion compensation has been suggested in the articles: B. Eggleton, et al., Electronics Letters, vol. 32, p. 1610, 1996; N. Litchinitser, et al, Journal of Lightwave Technology, vol. 15, pp. 1323-1328, 1997; and N. Litchinitser, et al., Journal of Lightwave Technology, vol. 15, pp. 1303-1313. These papers demonstrated the effectiveness of this technique for a single, fixed wavelength in a particular system geometry. Examples of the power transmission and dispersion versus wavelength for a uniform grating are shown in FIGS. 3 and 4. At wavelengths slightly longer than the long-wavelength reflection band edge of the grating, negative dispersion is produced, while at wavelengths below the short-wavelength reflection band edge, positive dispersion is provided. For large dispersion magnitudes (greater than 1000 ps/nm), a large index modulation (greater than 0.001) and a long grating (greater than 10 cm) are typically required. In addition, proper apodization (i.e., smoothing of the profile at the grating edges) is necessary to remove sidelobe structure present in a uniform grating. The graphs in FIGS. 3 and 4 are for a grating of 10 cm in length with a refractive index of 0.0018 and super-Gaussian apodization. It should be emphasized that dispersion compensation by chirped gratings is achieved through reflection, whereas dispersion compensation by uniform gratings is achieved through transmission, two very different mechanisms that exhibit markedly different physics.
A related technology is that of strain-tuned fiber gratings. It is known that the resonant wavelength of an individual fiber grating may be tuned by either tensile strain (i.e., stretching) or compressive strain. Several techniques have been proposed for producing tensile strain, including thermal and piezo-electrical mechanisms. U.S. Pat. No. 5,007,705, issued Apr. 16, 1991 and U.S. Pat. No. 5,469,520, issued Nov. 21, 1995 describe tensile and compressive strain tuning, respectively, and are incorporated herein by reference. Strain tuning has been applied to a uniform grating used for filtering and to a chirped grating used for dispersion compensation.
Present dispersion compensation methods, as described above, have several shortfalls. Dispersion compensators formed by long lengths of compensating fiber are cumbersome and their properties can only be changed in discrete steps since change is accomplished by switching lengths of fiber in and out of the compensator. A chirped fiber Bragg grating has a narrow bandwidth, and even if strain tuned, is only adjustable over a small range. Additionally, a chirped grating typically requires a length on the order of meters for full compensation. A uniform fiber Bragg grating only provides dispersion when the incident wavelength is just outside a reflection band edge, and therefore, it is difficult to manufacture such a grating precisely enough to provide the required compensation, and difficult to maintain the dispersive characteristic, even with strain tuning. What is needed is a way to provide a reliable, fully adjustable (tunable), broadband dispersion compensator with a wide dynamic range. Additionally, such a compensator could be enhanced through the development of a monitoring and control system that monitors dispersion asynchronously and controls dispersion compensating elements using relatively inexpensive hardware.